The present invention relates to a method and apparatus using ion chromatography (“IC”) in which the suppressed analyte is converted to a salt prior to detection.
Ion chromatography is a known technique for the analysis of ions which typically includes a chromatographic separation zone using an eluent containing an electrolyte, and an eluent suppression stage, followed by detection, typically performed by a conductivity detector. In the chromatographic separation stage, ions of an injected sample are eluted from a separation column. In the suppression stage, electrical conductivity of the eluent electrolyte is suppressed but not that of the separated ions. In the first generation of ion chromatography, suppression or stripping of electrolyte used an ion exchange resin bed. In an improved form of suppression, a charged membrane in the form of a fiber or sheet is used in place of the resin bed. In sheet form, the sample and eluent are passed on one side of the sheet with a flowing regenerant on the other side of the sheet. The sheet comprises an ion exchange membrane partitioning the regenerant from the effluent of chromatographic separation. The membrane passes ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to weakly ionized form, followed by detection of the ions.
One effective form of suppressor is described in U.S. Pat. No. 4,999,098. In this apparatus, the suppressor includes at least one regenerant compartment and one chromatographic effluent compartment separated by an ion exchange membrane sheet. The sheet allows transmembrane passage of ions of the same charge as its exchangeable ions. Ion exchange screens are used in the regenerant and effluent compartments. Flow from the effluent compartment is directed to a detector, such as an electrical conductivity detector, for detecting the resolved ionic species. The screens provide ion exchange sites and serve to provide site to site transfer paths across the effluent flow channel so that suppression capacity is no longer limited by diffusion of ions from the bulk solution to the membrane. A sandwich suppressor is also disclosed including a second membrane sheet opposite to the first membrane sheet and defining a second regenerant compartment. Spaced electrodes are disclosed in communication with both regenerant chambers along the length of the suppressor. By applying an electrical potential across the electrodes, there is an increase in the suppression capacity of the device. The patent discloses a typical regenerant solution (acid or base) flowing in the regenerant flow channels and supplied from a regenerant delivery source. In a typical anion analysis system, sodium hydroxide is the eluent and sulfuric acid is the regenerant. The patent also discloses using water to replace the regenerant solution in the electrodialytic mode. In an improved form of membrane suppressor, described in U.S. Pat. No. 5,352,360, effluent from the detector is recycled through the regenerant flow channels.
In Berglund, I., et al. Anal. Chem. 63: 2175 (1991), another multiple detector system is described. Here, conventional IC is performed using a first conductivity detector. The effluent from that detector is passed sequentially through cation exchange and anion exchange conversion zones. For anion analysis, the effluent from the first detector is in the usual IC form of HX (wherein X is the analyte anion) as it exits from the suppressor. Two different types of convertors are disclosed. In a sequential packed column form, the effluent first passes cation (sodium) exchange resin and then anion (hydroxide) exchange resin, resulting in sequential conversion first to NaX salt and thereafter to NaOH. A permselective membrane-type convertor is also disclosed for such sequential conversion. After conversion, the ion conductivity of the sodium hydroxide is measured in the second detector and compared to the ion conductivity of the first detector. The paper states that the data reveals peaks due to very weak acids hidden in the suppressed base line or overlapped with strong acid peaks. It further states that this method allows an estimation of the pK of the analyte peak and permits approximate quantitation without standards. Problems with that system include the following: (1) incomplete conversion of the acid form analyte to NaOH due to differences in ion exchange selectivity between hydronium and sodium, and analyte anion and hydroxide on the cation and anion exchange resins respectively; and (2) analyte band dispersion in the ion exchange columns must be compensated for when ratioing the signals from the two detectors. For weak acids, for example, it can be more of a problem, because there is less free hydronium ion available to exchange for sodium ion.
In PCT Publication WO 9418555, apparatus and methods are disclosed using IC principles in which different detectors provide useful comparative signals. Specifically, in one form of the apparatus, separating means, typically in the form of a chromatographic resin column, separates the analyte ions in the presence of an eluent comprising electrolyte. The effluent from the separating means flows through suppressor means for converting the electrolyte to weakly ionized form and the analyte ions to acid or base form. The suppressed effluent flows through a first detector for detecting the conductivity of the ionic species and generates a first signal. This portion of the system is conventional suppressed IC. The effluent from the first detector flows through a salt convertor for converting the analyte ions in acid or base form and to salt form. Then, the conductivity of the salt form of the analyte is measured in a second detector means and a second signal is generated. The first and second signals are analyzed to represent a defined relationship between the output signals.
In one embodiment of WO 9418555, the analyte ions in acid or base form are converted to their corresponding salts in a single conversion with salt-forming ions of opposite charge. For example, for analyte anions represented by “X”, and using Na+ ion, NaX is measured in the second detector means. This is referred to herein as the “single conversion mode.” It discloses a salt convertor which minimizes dispersions which could skew peak ratios of the single conversion type. One disclosed single conversion convertor is an on-line microelectrodialytic ion source which supplies the salt-forming ion through a membrane. It includes a salt-forming ion source channel, a suppressor effluent flow channel and a permselective ion exchange membrane partitioning the two channels. The membrane includes exchangeable ions of the same charge as the salt-forming ions and is resistant to transmembrane passage of the ionic species. An electrical potential is applied between the ion source channel and suppressor effluent flow channel. The latter channel is in fluid communication with the effluent from the suppressor. In operation, the signal generated in the first conductivity detector for the acid or base form of the analyte is evaluated with the signal generated in the second ion conductivity detector for the salt form of the analyte to provide extremely useful information. Other disclosed single conversion convertors include the use of an ion exchange membrane barrier without electrolysis, but with external acid or base concentrations sufficient to overcome the Donnan barrier. Still other systems include the use of a porous membrane barrier using the application of current or differential pressure to drive the acid or base salt-forming ions into the suppressor effluent flow channel. Single conversion is also disclosed by flowing the suppressor effluent stream through an ion exchange medium such as a column of an ion exchange resin bed having exchangeable ions of opposite charge to the analyte ions.
WO 9418555 also discloses a “double conversion mode” in which the analyte ions are twice-converted. In this instance, the analyte ion is converted to a salt of (a) the same type of counterion as in the single conversion mode, and (b) a common single ion of the same charge as the analyte ion by simultaneous ion exchange of the acid of base form of the analyte ions with the selected anion and cation. In one embodiment using a permselective membrane, the suppressor effluent flows in a central channel flanked by two ion source channels, one including anions and the other including cations. Permselective membranes separate the ion source channels from the suppressor effluent flow channel and include exchangeable ions of a type which permit transport of such cations and anions into the suppressor effluent flow channel to accomplish double conversion. In another simultaneous double conversion, the suppressor effluent flows from the first detector through ion exchange medium such as an ion exchange resin bed, including exchangeable anions and cations of the same type desired as in the permselective membrane. Sequential double conversion is also disclosed. In one embodiment, the suppressor effluent flows from the first detector sequentially through two ion exchange columns of opposite charge. For example, the first column includes a common, single ion of the same charge as the analyte ions so that a converted acid or base with a common anion or cation is formed in the first column which is passed to the second column for conversion to a salt, or the order of the columns may be reversed. Also, it discloses a permselective membrane system for the sequential double conversion embodiment.
Another attempt to convert suppressed chromatography effluent to a salt using a membrane suppressor in the chemical mode with countercurrent flow is disclosed in Yuan Huang, Shi-fen Mou, Ke-na Liu, J. Chromatography, A 832:141-148 (1999). In this approach, only enough regenerant solution was provided so that suppression was incomplete. However, it is difficult to control the background and noise. The device is extremely sensitive to both the regenerant flow rate and the eluent flow rate for a given regenerant concentration.
U.S. Pat. No. 4,455,233 discloses another approach to salt conversion, using an eluent with an acid or base with a co-ion of the same charge as the ions analyzed, in which the co-ions being in the hydronium or hydroxide form. In this approach, the electrolyte for anions is an acid and the eluent for the cation is a base. Both the eluent and the analyte are converted to salt form. Although the eluent has a lower conductivity in the salt form than the conductive form, the background in this approach can be as high as 100 US/cm. Such high backgrounds result in higher chromatographic noise. The above approach is generally not compatible with commonly used eluents for ion chromatography and require eluents that readily get converted to the salt form of lower background.
There is a need in suppressed chromatography for efficient systems to convert weakly dissociated analytes into salt form and to facilitate detection of such analytes or subsequent reaction products against a low background.